Humans are likely far less intelligent/capable than systems that will soon be practically feasible to engineer, and we will lose to a generally superhuman AI that has sufficiently different objectives than we do. So an accident with a runaway a system like that would be catastrophic.

Then Yudkowsky spins a gauntlet of no-evidence hypotheses for why such an accident is inevitable and leads to the death of literally all humans in the same instant.

But the first part of the argument is something that will be the critical piece of the engineering of such systems.